Manipulation of time-dependent multicolour evolution of X-ray excited afterglow in lanthanide-doped fluoride nanoparticles

External manipulation of emission colour is of significance for scientific research and applications, however, the general stimulus-responsive colour modulation method requires both stringent control of microstructures and continously adjustment of particular stimuli conditions. Here, we introduce pathways to manipulate the kinetics of time evolution of both intensity and spectral characteristics of X-ray excited afterglow (XEA) by regioselective doping of lanthanide activators in core-shell nanostructures. Our work reported here reveals the following phenomena: 1. The XEA intensities of multiple lanthanide activators are significantly enhanced via incorporating interstitial Na+ ions inside the nanocrystal structure. 2. The XEA intensities of activators exhibit diverse decay rates in the core and the shell and can largely be tuned separately, which enables us to realize a series of core@shell NPs featuring distinct time-dependent afterglow colour evolution. 3. A core/multi-shell NP structure can be designed to simultaneously generate afterglow, upconversion and downshifting to realize multimode time-dependent multicolour evolutions. These findings can promote the development of superior XEA and plentiful spectral manipulation, opening up a broad range of applications ranging from multiplexed biosensing, to high-capacity information encryption, to multidimensional displays and to multifunctional optoelectronic devices.

-Figure panels: the figure panels are of high quality and attractive. I think they could be further improved if adding labels for used Ln ions or concentrations to allow the reader to identify these critical parameters at a glance rather than looking for them in captions or text.
-What are the dimensions of the phosphor spots photographed (a scale bar would be helpful)? Are those photographs of dry powders or dispersions? At which time point where the photos taken? - Figure 2c: The linear lines may be misleading towards a linear correlation between ratio and intensity (which seems not to be the case based on intensities seen in Fig. 2b).
-Multi-color modulation: How fast do these color changes happen? Providing color points at shorter time intervals may be helpful. o "time-dependent color modulation on demand" -the color change seems to be dependent on the decay of the X-ray induced afterglow. How can this be controlled on demand? o Figure S3: the caption should probably also include 12.5. o Na3HfF7:Yb/Er and NaLuF4:Yb/Ho are mentioned -how where these obtained and what are their sizes / size distributions? o I could not find information about instrumentation used for DS and UC.

Reviewer #3 (Remarks to the Author):
This manuscript is concerned with the time dependent multicolour evolution of x-ray afterglow from lanthanide doped fluoride nanoparticles. This is mainly achieved by using different lanthanide ions as activators as they control the rate of afterglow decay. Then by having different compartments (core/shell) one has one part of the NP glowing in one color, the other in another color. By having the color intensity decay at different time scales leads to a changing color observed. This color effects of the NPs can be further modified by illuminating them with NIR or UV light, in order to trigger upconversion (UC) or downshifting (DS), respectively. In general, this is interesting work and the authors also performed a comprehensive and careful characterization work on their nanoparticles and their photophysical properties. This was done for a systematic variation of the composition of the NPs, which is another strength of this article. Interesting here is certainly the observation that they can increase substantially the emission intensity by incorporating Na+ ions. In summary, a number of interesting optical properties are demonstrated in this work. However, one crucial point regarding the chosen architecture of these core-shell or core-shell-shell nanoparticles still must be explained. This concerns the explanation, why the construction of such rather complex NPs is necessary at all to arrive at the reported phenomena. In principle, many of them should also be achievable by simply mixing the individual NP with the materials contained in the core and the shell (or shells), isn't that the case. Therefore, the central point to be made is to explain what is the gain from having these different materials contained within one more complex NP that requires correspondingly more synthetic effort. In Figs. 4 and 5 the change of the colors is shown visually. Interesting here could also be to see the full emission spectra as a function of time.
Interesting would also to know how high is the percentage of the energy of the light emitted by the NPs compared to the energy put in via x-ray radiation. As a minor point on page the units of the crystal lattice should be given. Finally, the use of the English language should be improved at various places, best by having this done by a native speaker. In summary, I think that this manuscript has the potential for becoming published but the crucial point for that would be a convincing explanation, why the structure of the NPs employed here is superior to simply a mixture of individual NPs.
We greatly appreciate the reviewers' insightful comments which are very helpful for improvement of our manuscript. In response to the valuable comments raised by the referees, we provide point-by-point responses along with the modifications (marked in blue) made in the revised manuscript. which is also focused on X-ray-charged bright persistent luminescence in NaYF4:Ln 3+ @NaYF4 nanoparticles for multidimensional optical information storage.
Although in the submitted paper the kind of fluoride materials are slightly different (NaLuF4 instead of NaYF4), a core@shell architecture and several combinations of luminescent lanthanide ions are similarly considered. Moreover, in the abovementioned paper (by Zhuang et al.), optical information-storage application is also demonstrated by inkjet based technology of the investigated nanoparticles.
Therefore, I think that the paper does not contain a very high novelty to be considered for publication in Nature Communications. I suggest to submit to Light: Science &

Applications.
Response: We appreciate the referee for this comment and we have read the suggested reference (Light: Science & Applications (2021)10:132) carefully. Actually, the reported literature is focused on XEA of single-type lanthanide ions doped NaYF4:Ln 3+ @NaYF4 NPs. In contrast, our work is devoted to developing a brand-new mechanism for the enhancement of XEA as well as novel spectral manipulation processes via multiple lanthanide ions doping in core@multishells NPs.
Accordingly, a very creative time-dependent multicolour evolution has been successfully realized, and a mechanism for optical information-storage has been proposed as well, which is distinct from that described in the suggested reference,.
Detailed comparisons between the present work and the one in the suggested reference are given below.
Firstly, the suggested literature reported the XEA phenomenon in NaYF4:Ln 3+ @NaYF4. In the present study, however, we developed a novel feasible method to greatly enhance the XEA intensities of multiply doped (Pr 3+ , Tb 3+ , Dy 3+ and Sm 3+ ) nanophosphor by means of introducing interstitial Na + ions in both the NaLuF4 core and the shell layer, considering that the enhancement in XEA intensity is of great importance for potential application of XEA in many areas such as bio-imaging and optical information storage. In addition, the time-dependent XEA colour evolution via judicious core/shell doping design enables new applications in anti-counterfeiting and bio-sensing. Therefore, the concept proposed in the present study should not be taken as that in the suggested literature. Instead, the strategy regarding core/shell doping design is actually a brand-new development towards manipulation of the XEA performance.
Secondly, the suggested literature mainly studied the XEA of NPs with a single Ln 3+ activator. In contrast, systematical study on the incorporation of dual-activators into core-shell nanostructure has been performed in the present work in order to control time-dependent XEA spectral variations.
Thirdly, different strategies are adopted to demonstrate the application of XEA NPs in optical information storage. In the suggested literature, three different kinds of luminescent inks containing different NaYF4:Ln 3+ @NaYF4 NPs (Ln = Tb, Dy, and Ho) were applied to print overlapping patterns for 3D optical information storage. In such case, optical filters have to be used for the colour variation. The present work, utilizes the various XEA decay times for different Ln activators located in both the core and the shell layers to realize time-dependent colour evolution in XEA, which can further be manipulated on demand Finally, owing to the improved XEA performances of different lanthanide activators in the present work, the specially-designed core/multi-shells fluoride NPs can be employed to simultaneously realize XEA, UC and DS emissions. Therefore, upon various excitation, one single nanostrucrture can exhibit several different time-dependent colour evolution modes, which may enable creative applications for optical information storage; this has not been reported in previous literatures.

Reviewer 2:
Comment: The manuscript "Manipulation and amplification of time dependent multicolour evolution of X-ray excited afterglow in lanthanide doped fluoride nanoparticles" by S. Xu an co-workers reports the synthesis and characterization of persistent nanophosphors based on lanthanide doped NaLuF4 nanoparticles (a structure that is very similar to that reported by F. Zhang, Nature Nanotechnology 16 (9), 1011-1018). Adjustment of the [Na]/[RE] ratio as well as the dopant's concentration and positioning within core-(multi)shell structures allow to tune the X-ray induced afterglow colour of the nanoparticles and adds additional downshifting and upconversion features. Combination of Ln dopants with different decay times and additional UV or NIR excitation results in time-dependent colour modulation. These nanophosphors may be interesting for application in advanced anti-counterfeiting, as proposed by the authors. Additionally proposed applications include biomedicine, optoelectronic devices and displays -which is not fully clear to me and which should be elaborated in more detail (e.g. how would the time-dependent change of emission colour benefit biomedical application? how would a display the colour of which is determined by the decay time of the persistent phosphor be suitable for rapid colour changes of pixels?). As such, the photophysics are very interesting, yet practical application potential, as highlighted by the authors, is vague.

Response:
We thank the reviewer's valuable comments and insightful suggestions.
Taking into account this comment, the promising applications described in the last sentence of "Discussion" section are slightly modified to "biomedicine, advanced anti-counterfeiting, displays, optoelectronic devices and potentially many others". To further elaborate these potential applications, more descriptions are added in the revised supporting information. Biomedicine: XEA can be used to monitor the drug release by coating drugs with absorption at specific wavelengths on the surface of NPs (XEA colour variation can vary before and after drug release). For example, after coating Doxorubicin or Daunorubicin (with absorption wavelength at a green region, Chem. Mater., 2017, 29, 7615-7628;Chem. Sci., 2018, 9, 3233) on the Tb@Sm or Tb@Pr NPs, XEA of the composites consists of mainly red emissions. Upon release of the coated drugs, the XEA colour will change from red to green ( Figure R1). Furthermore, since the XEA can be used to judge if a drug is released and the UC process can be used for bio-detection (ACS Nano, 2021, 15, 3709-3735;Biomaterials Advances, 2022, 136, 212763), a combination of XEA and UC might be used to guide the bio-detection. In addition, persistent phosphors show promising applications in bio-imaging and photodynamic therapy (Adv. Funct. Mater. 2018, 1707496;Nature Nanotechnology, 2021, 16, 1011-1018ACS Nano, 2019, 13, 10419-10433;Nano Lett. 2019, 19, 8234-8244;Nano Energy, 2021, 79, 105437). In our work, the improved XEA performance of the lanthanide doped fluoride NPs is believed to facilitate their application in bio-imaging. Anti-counterfeiting: Multicolour NPs for advanced anti-counterfeiting has been verified in our work as well as in many other reports (Nature Communiations, 2017, 8, 899;Adv. Mater. 2019, 1901430;Small, 2020, 2000708;Adv. Funct. Mater. 2021, 31, 2009920;Adv. Optical Mater. 2019, 1900519). Display: By increasing the X-ray irradition power or time, the afterglow time can be prolonged to hours in our case (Fig. S14). Moreover, in the previous reported literature (Nature, 2021, 590, 410), the XEA of lanthanide doped NPs can last more than 30 days. Thus, through integrating XEA with UC and DS, the emission color switch can be feasibly achieved by tuning the NIR or UV excitation power with controlled time intervals. The output colour can be kept changed in hours, which could find applications for display. Optoelectronic devices: the lantanide doped nanoscintillators featuring both XEA and XEOL can be used as a flexible detector to realize flat-panel-free X-ray imaging of 3D electronic objects (Nature, 2021, 590, 410). In our case, the enhancement in emission intensity for both the XEA and XEOL should improve the imaging quliaty. In addition, the core/shell/shell NPs can respond to X-rays, UV and NIR photons simulteneously, which provides a chance to develop broadband photodetectors. Considering that the length of the revised manuscript may be too long, this peer-review response will be provided online and only the following simplified contents have been added in the conclusion section of the revised manuscript.
These results not only provide a facile avenue for developing X-ray activated afterglow nanomaterials with superior performances, but also to build a general platform to modulate the emission colour in fluoride core/shell NPs, which may find promising applications in biomedicine (i.e., biosensing, bio-imaging and photodynamic thereapy), advanced anti-counterfeiting, displays, optoelectronic devices (i.e., broadband photodectors) and potentially many others.

Comment: Incorporation of Na ions: the nominal ratio of [Na]/[RE] is significantly
increased whereas the actual increase is very small (Table S1). How was the [Na] determined? Can such small increase in [Na] within the NPs indeed induce such strong XEA enhancement; any theoretical approach that would support this?

Response:
The variation trends of [Na] concentration was first revealed by energy dispersive X-ray spectroscopy (EDS) and further determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES). As shown in Figure R2, the EDS analysis reveals that the Na signal enhances gradually with an increase of the [Na] concentration in the precursors solution. The weight ratio of the cations was measured by ICP-OES and the corresponding molar ratio was calculated (  The Rietveld XRD refinement, energy dispersive X-ray spectroscopy, inductively coupled plasma-optical emission spectroscopy and X-ray photoelectron spectroscopy results have been analyzed in our original manuscript, which indicates that interstitial Na + ions are formed in the NaLuF 4 crystal structure. The interstitial Na + ions not only decrease binding energies of the Fions, but also bring electrostatic interactions between the Fand interstitial Na + ions. Thus, in the case of high [Na]/[RE] ratio, the anion Frenkel defects are easier to be formed upon X-ray irradiation via elastic collisions between large-momentum X-ray photons and small fluoride ions, benefiting the formation of traps. To further verify the proposed mechanism, the formation energy of anion Frenkel defects in the NaLuF4 crystal structure was calculated based on the density functional theory (DFT) with the projector augmented plane-wave method. The crystal structures of the original 2 x 2 x 1 supercell of NaLuF4 and optimized interstitial Na-doped NaLuF4 are shown in Figure S22a-b. The anion Frenkel defect formation energies Ef for the dislocation of Fions into interstitial sites with different separation distances (0.5, 1.0, 1.5, 2.0 8 -70 )%* F# ?1;1 /-5/>5-=10. As shown in Figure S22c, after the incorporation of interstitial Na + ions, the Ef for the Frenkel defects with various separation distances are consistently reduced. Furthermore, the Ef values for different interstitial Na + sites were calculated by considering the relaxation of all atoms owing to the introducing of interstitial Na + ions and formation of Frenkel defects ( Figure S22d-e).
The amount of X-ray induced Frenkel defects (nF) can be expressed by the following equation 5 : where Nl and Ni are the number of Flattices and interstitial sites, respectively, k and T are Boltzmann constant and temperature, respectively. In this occasion, the decrease of Ef can lead to an evident increase of nF, which benefits the formation of high concentration traps.

Comment:
What is the reason for the XEA loss at a ratio of 12.5?  Fig. S8 in the next comment). The results show that the XEA intensity was indeed increased after the removal of NaF ( Figure R3 in the next comment). Based on these results, it is suggested that the existence of non-active NaF can reduce the total number of emitters so as to weaken the XEA intensity. Meanwhile, the high [Na]/[RE] ratio induced excessive anion Frenkel defects can serve as quenching sites to decrease the XEA intensity via non-radiative relaxations.

Response
The following contents have been added in the revised manuscript.
For the [Na]/[RE] ratio above 10, the non-active NaF reduces the total number of emitters and the excessive anion Frenkel defects might promote non-radiative relaxation. Both effects contribute to the decrease of the XEOL and XEA intensities.
Comment: NaF formation: NaF can easily be washed away with a water-ethanol mixture. XEOL and XEA data were obtained on dry powders? If parts of these are indeed NaF, that may simply have an effect of how many X-ray-activated NPs there are and how many emitters compared to non-active NaF? How do the samples perform after washing to remove NaF?
Response: According to the reviewer's helpful suggestions, the NaLuF4: Gd/Tb NPs prepared with different [Na]/[RE] (2.5, 5, 7.5, 10 and 12.5) and [F]/[RE] of 5 were first treated with diluted HCl and then washed with water-ethanol mixture. In our original manuscript, we have clarified that the NaF phase emerged when the [Na]/[RE] ratio was 7.5, 10 or 12.5. As shown in Fig. S8, the excessive NaF (obtained with the conditions of [Na]/[RE] = 7.5, 10 and 12.5) was removed by washing with water-ethanol mixture. In this case, the XEA intensities of these pure phases without NaF were compared, where the XEOL/XEA data was obtained on dry powders. To estimate the ratio of emitters to non-active NaF and the corresponding influence on the XEA intensity, the EDS and XEA spectra of the NaLuF4: Gd/Tb NPs ([Na]/ [RE] of 10) before and after washing with water-ethanol were studied. Through comparing the [Na] content before and after washing with water-ethanol based on the EDS results, the NaF content was about 23 % for [Na]/[RE] = 10. The XEA intensity was indeed slightly increased after the removal of NaF ( Figure R3). However, as shown in   Response: According to the reviewer's helpful comment, the afterglow duration for the NaLuF4:Tb and commercial SrAl2O4:Eu,Dy products were compared, and the XEA decay times for the NaLuF4:Tb (5, 10, 15, 20 mol%), NaLuF4:Dy (0.5, 1, 2 mol%), and NaLuF4:Pr (0.5, 1, 2 mol%) NPs with different activators concentrations were systematically studied.
In our original manuscript, a spectrometer with PMT-CR131-TE detector (OmniFluo-Xray-JL) was used to test the XEA performances of those lanthanide doped fluoride NPs. To better measure the afterglow time of the present studied NaLuF4:Tb NPs, a better spectrometer (FLS980, Edinburgh Instrument) with R928 detector was employed. As shown in Fig. S14, under the same X-ray irradiation and measurement conditions, both the XEA intensity and afterglow time of the present studied NaLuF4:Tb NPs are superior to that of commercial SrAl2O4:Eu,Dy persistent phosphor. The measured afterglow time of the NaLuF4:Tb NPs was actually up to more than 6 hours.  NaGdF4:Yb/Tm@NaGdF4:X (X= Tb, Eu, Dy, or Sm) (Nature Materials, 2011, 10, 968), it has been verified that the Gd 3+ ions can promote the energy migration from Gd 3+ : 6 P7/2 to Tb 3+ , Eu 3+ , Dy 3+ and Sm 3+ activators. Moreover, the 6 P7/2 state and ground state of Gd 3+ are separated by a relatively large energy gap (~3.2*10 4 cm -1 ), leading to a minimized energy loss caused by multiphonon emission and cross-relaxation (Science, 1999, 283, 663-666). As such, the Gd 3+ ions have been widely employed to enhance luminescence emission intensities. In the X-ray excited optical luminescence field, it was verified that "at a low Gd 3+ concentration, the excitation energy can be efficiently transferred from Gd 3+ to Tb 3+ activators. By comparison, at a high Gd 3+ concentration, the excitation energy dissipates non-radiatively to quenching sites through energy migration, resulting in fast spontaneous emission of Tb 3+ with low afterglow intensity. (Nature, 2021, 590, 410)".
In our work, to further improve the XEA intensity via introducing energy migration process, the Gd 3+ ions were codoped with various activators. As shown in Figure S9-10, both of the XEOL and XEA intensities were significantly increased with Gd 3+ doping ratio of 15 mol%. Moreover, with increasing the Tb 3+ ions concentration from 5 to 20 mol% in the NaLuF4: 15Gd/Tb NPs (Figure S11), the Gd 3+ emission at ~311 nm corresponding to its 6 P7/2C 8 S7/2 transition was evidently decreased gradually, which further verify the existence of energy transfer from Gd 3+ : 6 P7/2 to Tb 3+ .
Based on the above discussion and additional experimental results, the following sentences were added in the "Spectroscopic study of afterglow intensification." section of the revised manuscript.
With increasing the Tb 3+ concentration, the Gd 3+ : 6 P7/2C 8 S7/2 emission intensity decreases (Fig. S11), revealing that Gd 3+ can promote the energy migration from its between 6 P7/2 and the ground state of Gd 3+ is in favor of minimizing energy loss caused by multiphonon assisted non-radiation relaxation and cross-relaxation 42 . In addition, the references of (Nature Materials, 2011, 10, 968 and Adv. Mater., 22, 3266) were added along with the sentence of "It should be noted that the incorporation of Gd 3+ ions with an optimal concentration of 15 mol% was used to facilitate the population of Tb 3+ excited levels via energy transfer from Gd 3+ to Tb 3+ followed by the improved XEOL and XEA intensities ( Fig. S8-9)." Comment: Core-shell architectures: It is highlighted that XEA can be observed selectively from core or shell. Are core-shell structures truly needed? Would the same effect be observed when doping Ln ions for XEA together in the core? I am further wondering why the author chose to dope the UC emitters into the outer shell: this is the most sensitive location for upconverters given surface quenching; the core might have been the more intuitive region for UC dopants.
As shown in Fig. S32a We agree that the upconverters are sensitive to surface quenchers, especially in the small NPs. In our original manuscript, the Tb@Ce/Sm@Yb/Tm core@shell@shell NPs were used as an example to show the colour editing process.
Actually, the change of activators distributions in the core@shell@shell structure may lead to different colour variation processes. To reveal the influence of Yb/Tm location on the UC intensity, the Tb@Ce/Sm@Yb/Tm, Tb@Yb/Tm@Ce/Sm and Yb/Tm@Tb@Ce/Sm core@shell@shell NPs were prepared and studied. As shown in Fig. S39, the UC intensity is enhanced when Yb/Tm are located in the core owing to surface passivation, while the XEA intensity is evidently decreased. The XEA intensity decreases over time, while the UC intensity is stable and can be tuned by changing the excitation power. In this case, a stronger initial intensity for XEA is in favor of the realization of multicolour evolution. The XEA intensities were greatly reduced when codoping dual-activators (Tb/Sm, Tb/Pr or Sm/Dy) in the core layer ( Fig. S32), indicating the spatial separation of activators in a core@shell structure is of importance for the realization of bright multicolour evolution.
After exchanging the NaLuF4:15Gd/15Tb and NaGdF4:49Yb/1Tm layer in the core@shell@shell structure, the UC intensity was enhanced while the XEA intensity was greatly reduced (Fig. S39).
Comment: Size and size distributions: standard deviations should be provided. Shell thicknesses should be included.
Response: According to the reviewer's kind comment, the standard deviations of the size distributions of the core and shell thicknesses have been provided in Fig. S1 and S24 in the revised supporting information.  Response: As suggested, the scale bar has been added in the revised Fig.2, Fig.4, Fig.   5 and Fig.6 in the revised manuscript. The circular photographs were recorded by loading the dry powders in a sample holder. After turning off the X-rays, the first photograph was recorded immediately (shorter than ~3 seconds) by iPhone.
Comment: Figure 2c: The linear lines may be misleading towards a linear correlation between ratio and intensity (which seems not to be the case based on intensities seen in Fig. 2b).
Response: According to the reviewer's kind suggestion, the linear lines shown in     ions. The high concentration Yb 3+ captures many electrons from the traps, and the efficiency of energy transfer from Yb 3+ to Tm 3+ is low in the XEA process (its mechanism is quite different from that of UC). Thus, the electron population in the Tm 3+ is greatly reduced after the incorporation of Yb 3+ ions. Therefore, in the NaLuF4:15Gd/15Tb@NaLuF4:15Gd/10Ce/0.5Sm@NaGdF4:49Yb/1Tm NPs, the XEA intensity is very weak in the NaGdF4:49Yb/1Tm layer as well. It should be noted that the Ce 3+ ions were employed to absorb the 254 nm UV photons and inhibit Sm 3+ XEA (Fig. S37), and the Yb 3+ ions were used to absorb the 980 nm NIR photons and inhibit Tm 3+ XEA (Fig. S38).
The Ce 3+ , Gd 3+ and Sm 3+ ions compete to capture electrons from the traps.
Compared with the parity forbidden transitions within the f-manifold of lanthanides, the 4f n -4f 7E( 5d 1 optical transitions are often characterized by high radiative emission probability because the f-d transition is electrical-dipole allowed. In our case, the Ce 3+ ion exhibits much larger absorption cross-section via 4f-5d transition than those of Gd 3+ and Sm 3+ ions via 4f-4f transitions. Thus, most of the electrons deposited in the traps will be captured by Ce 3+ ions. Although the energy transfer processes from Ce 3+ to Gd 3+ and then to Sm 3+ has been widely used to produce DS emission, it is hard to avoid energy loss during the energy transfer from Ce 3+ to Gd 3+ and from Gd 3+ to Sm 3+ .
As a result, the incorporated Ce 3+ ions will capture large number of electrons deposited in the traps, many of which are lost during the energy transfer processes.
The high concentration Yb 3+ captures many electrons from the traps, and the efficiency of energy transfer from Yb 3+ to Tm 3+ is low for XEA process. Thus, the electron population in the Tm 3+ is greatly reduced after the incorporation of Yb 3+ ions.   Edit., 2021, 133, 22427-22433), we did not compare the XEA performance between carbon dots and NaLuF4:Tb NPs. As shown in Fig. S13, the initial XEA intensity of the NaLuF4:Tb NPs was much stronger than those conventional persistent phosphors upon the same X-ray irradiation conditions and PMT detector. Even after 20 mins, the XEA intensity of the NaLuF4:Tb NPs remained much stronger than the others. The XEA intensity is stronger than previously reported Sr13Al22Si10O66:Eu and commercial SrAl2O4:Eu/Dy, ZnS:Cu, ZnS:Cu/Co and CaS:Eu persistent phosphors ( Fig. S13), and the afterglow time can last up to more than 6 hours (Fig. S14). Inset shows their corresponding normalized initial XEA intensities.

Comment
The cited references for those conventional persistent phosphors are published by different research groups over the world including Germany, America, China, France, Singapore and Russia.

Comment:
There are a few typos and sentences that may need clarification: Response: Thanks for the reviewer's careful reading.
The "line 24 (the a)" was corrected to "the"; The "line 185 (produce)" was corrected to "benefits" Comment: what is meant by "the afterglows exhibit same excitation and emission Response: To show that the time-dependent colour modulation can be controlled on demand, a schematic illustration is plotted to reveal the colour variations. As shown in Figure R4, the afterglow contains green and red colors (a), and the green one decays slower than the red one (b). In this case, if the initial intensity of the red colour is much stronger than the green one, then the output afterglow will be changed from red to green (c); if the initial intensity of the red colour is similar to the green one, then the output afterglow will be changed from yellow to green (d); if the initial intensity of the red colour is weaker than the green one, then the output afterglow will be green (e). This is only a simplified illustration. In our case, the Tb (green), Dy (cyan) and Pr

Figure R4
Schematic illustration of XEA colour variations at different conditions. Comment: Figure S3: the caption should probably also include 12.5.
Response: The 12.5 has been added in the revised Fig. S3. Response: According to the reviewer's kind comment, the preparation methods and their SEM images as well as size distributions for the Na3HfF7:Yb/Er and NaLuF4:Yb/Ho products are provided in the revised manuscript. It should be noted that the as-prepared Na3HfF7: Yb/Er exhibited much broader size distributions (from ~120 to ~320 nm), which did not provide owing to the big error.
The following contents and Fig. S42 has been added in the revised supporting information. Afterwards, 8 mL of methanol solution containing NH4F (3 mmol) and NaOH

Synthesis of lanthanide-doped
(2 mmol) was added into the above solution and stirred at 60 o C for 40 min.
After the methanol was fully evaporated, the solution was heated to 290 o C under N2 atmosphere and kept for 90 min, and then cooled down to room temperature. The products were precipitated by centrifugation.

Comment: I could not find information about instrumentation used for DS and UC.
Response: According to the reviewer's kind comment, the information about instrumentation used for DS and UC has been added in the revised manuscript.
The following sentence of "The DS and UC emission spectra were recorded with a spectrometer (Edinburgh FLS980) equipped with an adjustable laser diode (980 nm, 0 -2 W) and Xenon lamp (450 W)." has been added in the Characterizations sections of the revised manuscript.

Reviewer 3:
Comment: This manuscript is concerned with the time dependent multicolour evolution of x-ray afterglow from lanthanide doped fluoride nanoparticles. This is mainly achieved by using different lanthanide ions as activators as they control the rate of afterglow decay. Then by having different compartments (core/shell) one has one part of the NP glowing in one colour, the other in another colour. By having the colour intensity decay at different time scales leads to a changing colour observed.
This colour effects of the NPs can be further modified by illuminating them with NIR or UV light, in order to trigger upconversion (UC) or downshifting (DS), respectively.
In general, this is interesting work and the authors also performed a comprehensive and careful characterization work on their nanoparticles and their photophysical properties. This was done for a systematic variation of the composition of the NPs, which is another strength of this article. Interesting here is certainly the observation that they can increase substantially the emission intensity by incorporating Na + ions.
In summary, a number of interesting optical properties are demonstrated in this work.

Response:
We thank the reviewer's insightful comments and appreciate for the recognition of our results. We have revised the following comments appropriately.

Comment:
One crucial point regarding the chosen architecture of these core-shell or core-shell-shell nanoparticles still must be explained. This concerns the explanation, why the construction of such rather complex NPs is necessary at all to arrive at the reported phenomena. In principle, many of them should also be achievable by simply mixing the individual NP with the materials contained in the core and the shell (or shells), isn't that the case. Therefore, the central point to be made is to explain what is the gain from having these different materials contained within one more complex NP that requires correspondingly more synthetic effort.
Response: According to the reviewer's kind comment, the XEA photogragh of the mixture NPs is recorded. As shown in Fig. S33, when simply mixing the NaLuF4:15Gd/15Tb and NaLuF4:15Gd/0.5Sm NPs, the red, yellow and green colours can be observed simultaneously and separately, indicating the XEA colour is not uniform.
The Fig. S33 and the sentence of "It should be noted that the physical mixture of different NPs will lead to the non-uniform colour distribution (Fig. S33)." have been added in the revised manuscript or supporting information.
Comment: In Figs. 4 and 5 the change of the colours is shown visually. Interesting here could also be to see the full emission spectra as a function of time.
Response: Thanks for the reviewer's kind suggestions, the XEA at different delay time for the Tb@Sm (Fig. S28), Tb@Pr (Fig. S30) and Sm@Dy (Fig. S31) NPs as well as UC (Fig. S41a) and DS (Fig. S41b) spectra at different times have been provided in the revised supporting information.   Comment: Interesting would also to know how high is the percentage of the energy of the light emitted by the NPs compared to the energy put in via x-ray radiation.

Response:
Thanks for the reviewer's kind comment. Generally, the pulse height measurement method is accurate to determine the light yield of a scintillator, and the decay time should be several ns or shorter. However, the lifetimes of the trivalent lanthanide ions are generally up to several ms, which is hard to be measured by the pulse height measurement method. To show the scintillation intensity of the present studied NaLuF4:Tb NPs, its XEOL intensity is compared with the commercial CsI:Tl and BGO scintillators. As shown in Fig. S12, the XEOL intensity of the NaLuF4:Tb NPs is about 0.42 and 2.17 times the strength of the commercial CsI:Tl and BGO scintillators, respectively. Comment: As a minor point on page the units of the crystal lattice should be given.

Response:
Thanks for pointing out. The unit (Å) of the crystal lattice has been added in the revised manuscript.
Comment: Finally, the use of the English language should be improved at various places, best by having this done by a native speaker.
Response: According to the reviewer's kind comment, we have checked the English language in the whole manuscript carefully and re-organized a few sentences appropriately in the revised manuscript.

Comment:
In summary, I think that this manuscript has the potential for becoming published but the crucial point for that would be a convincing explanation, why the structure of the NPs employed here is superior to simply a mixture of individual NPs.

Response:
We thank the reviewer's insightful comments again. The importance of the core/shell structure in our work has been studied and added in the revised manuscript or supporting information. We hope the reviewer agrees.

REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): The revised version has been notably improved and now the paper is worth to be considered for publication.
Reviewer #2 (Remarks to the Author): The revised version of the manuscript contains a significant amount of additional data and discussion, greatly contributing to the clarification of the questions that were previously brought up. Still, I have some remaining questions with respect to nominal versus actual Na-RE ion ratio, and a few minor comments (see below).
"High" Na-RE ratios are found to provide better performance in terms of XEA. However, the actual ratio as determined by ICP and presented in Table S1 seems not to be very high. Comparing nominal and actual values, the excess ratio of 10 only yields an actual ratio of 1.3. Adding Na in 2.5x excess, a minimal increase to a ratio of 1.096 was determined.
1. How were the actual ratios in Table S1 determined? The table provides information about RE% (but only relative to each other, summing up to 100%), but no information about Na%. Both would be needed to follow the calculation of the actual ratio.
2. If I understand it correctly, the "pure" lattice would have a Na-RE ratio of 1. Thus, only up to max 0.3 interstitial Na ions per RE ion are found in the lattice. Why is so little of the excess Na incorporated into the lattice? How many interstitial Na ions would this be per unit cell? 3. What is the fate of the excess Na not incorporated? XRD patterns do not show the formation of NaF unless applying the highest Na-RE nominal ratios. Is any amorphous secondary phase formed? 4. Taking into account the actual Na-RE ion ratios, please explain why the specific values for distance in the DFT calculations were used (and which distances specifically are referred to).
Minor comments: -Page 3, line 54: This sentence is difficult to understand. Do you mean something like maybe "...the afterglow exhibits a time-dependent intensity decrease, while the spectral profile remains unchanged."? -Page 4, line 67: "An innovative route that incorporating interstitial…" -this sentence is difficult to understand.
- Figure 2c: What were the intensities normalized to? Were the integrated intensities obtained on the spectra shown in Figures S16-S18? Or did you use the images in h) and i) to extract the intensity/brightness (using some software).
-Page 8, line 148: each of the lattice parameters should have a unit (or Angstrom^3).
-Page 12, line 207: The main text gives a particle size of ca. 43 nm, which seems not to be in agreement with the data show in the SI -average sizes given in Figure S24 are 60-65nm.
-Page 12, line 225: Lanthanide should be lanthanide -I find the figure caption of Fig. 4 confusing with respect to core-only or core-shell architectures. Sample names that clearly provide info about the architecture (as already used elsewhere) would be helpful.
-A very large number of various samples was prepared and analyzed; yet, only a fraction of them is described in the synthesis. In case that various samples were synthesized using the same approach but different RE dopants in core or shell, this should be mentioned for clarity. A table summarizing the samples with their sizes and architectures as well as key XEA, DS and UC features might also be helpful (only in the SI).
-Potential application of the materials -on demand XEA: It is called "on demand". But once XEA is triggered, is there any way to control the color evolution? Can it be stopped / slowed down / accelerated? Or will it just happen over the sample-specific timeline? Does it depend on the environment of the NPs (solvent, temperature, ...)? -I am not asking for such experimental assessment, but for how these probes are expected to work under real-life conditions; especially with respect to the highlighted "on demand".
-Potential application of the materials -bioimaging and biosensing as well as drug release monitoring: Taking into account that the emitted light falls into the visible spectral region, bioimaging of thicker samples / in-vivo seems not to be feasible to me. The suggested PDT, in contrast, seems to be promising to me, given that the samples could be excited outside the body and the PDT take affect afterwards, once admitted to the body.
-Experimental Section: was the precursor mixture stirred to remove water under ambient pressure or vacuum? -SI line 77: the amount of NaOA should be given in mol as for the other compounds.
-Size distribution histograms should include a y-axis to show how many NPs were considered in the size count.
-SI line 136: "NPs NPs" - Figure S5a and b: These are data for the ratio of 10 or 12.5? - Figures S9 and S10, b: what was the "normalized integral intensity" normalized to? One might expect a max value of 1 in case of a normalized data representation.
-Schematic illustrations of crystal lattices: addition of a color code for the different ions might be helpful.
Reviewer #3 (Remarks to the Author): In my opinion the authors did a very good job in replying to the concerns raised by the reviewers. Accordingly, I would then suggest publication of this work.
We greatly appreciate the reviewers' insightful comments which are very helpful for further improvement of our manuscript. In response to the valuable comments raised by the referees, we provide point-by-point responses along with the modifications (marked in blue) made in the revised manuscript.

Reviewer 2:
Comment: The revised version of the manuscript contains a significant amount of additional data and discussion, greatly contributing to the clarification of the questions that were previously brought up.
Still, I have some remaining questions with respect to nominal versus actual Na-RE ion ratio, and a few minor comments (see below).
"High" Na-RE ratios are found to provide better performance in terms of XEA.
However, the actual ratio as determined by ICP and presented in Table S1 seems not to be very high. Comparing nominal and actual values, the excess ratio of 10 only yields an actual ratio of 1.3. Adding Na in 2.5x excess, a minimal increase to a ratio of 1.096 was determined.

Response:
We thank the reviewer's insightful comments and appreciate for the recognition of our results. We have revised the following comments appropriately.
Comment: How were the actual ratios in Table S1 determined? The table provides information about RE% (but only relative to each other, summing up to 100%), but no information about Na%. Both would be needed to follow the calculation of the actual ratio.

Response:
The weight ratios of the cations were measured by ICP-OES, and then the corresponding molar ratio results were calculated (Table S1). To clearly reveal the Gd The revised Table S1 has been added in the revised supporting information. Why is so little of the excess Na incorporated into the lattice? How many interstitial Na ions would this be per unit cell?
Response: We agree with that the "pure" lattice has a Na-RE ratio of 1, and up to max 0.3 interstitial Na ions per RE ion is found in the lattice in our case. Firstly, most Na is  As revealed in the Fig. S5

Response:
The original sentence of "An innovative route that incorporating interstitial Na + ions inside the nanocrystal structure was employed to amplify the XEA intensities of the NaLuF4: Gd/(Pr, Tb, Dy or Sm) NPs." is corrected to "Incorporating interstitial Na + ions inside the nanocrystal structure was employed as an innovative route to amplify the XEA intensities of the NaLuF4: Gd/(Pr, Tb, Dy or Sm) NPs." in the revised manuscript.
Comment: Figure 2c: What were the intensities normalized to? Were the integrated intensities obtained on the spectra shown in Figures S16-S18? Or did you use the images in h) and i) to extract the intensity/brightness (using some software).

Response:
The integrated intensities were obtained in the spectra shown in Figures S3 and S16-S18. To clearly show the degree of XEOL intensification of the Pr, Tb, Dy and Sm activators after the incorporation of interstitial Na ions, the XEOL intensities for all the [Na]/[RE] = 2.5 were normalized to 1. It should be noted that Figure 2c cannot be used to compare relative XEOL intensities among different lanthanide activators.
Comment: Page 12, line 207: The main text gives a particle size of ca. 43 nm, which seems not to be in agreement with the data show in the SI -average sizes given in Figure S24 are 60-65nm.

Response:
The mean particle size shown in the main text corresponding to the Figure   S24 has been corrected to ~62±2 nm.
Response: Lanthanide shown in Page 12, line 225 has been corrected to lanthanide.
Comment: I find the figure caption of Fig. 4 confusing with respect to core-only or core-shell architectures. Sample names that clearly provide info about the architecture (as already used elsewhere) would be helpful.
In addition, for the paragraph of "As an example of practical multidimensional display application,…", the core-only NPs and core@shell@shell NPs were both emerged in the text, thus, the "core-only" has been added accordingly as well. Comment: Potential application of the materials -on demand XEA: It is called "on demand". But once XEA is triggered, is there any way to control the color evolution?
Can it be stopped / slowed down / accelerated? Or will it just happen over the sample-specific timeline? Does it depend on the environment of the NPs (solvent, temperature, ...)? -I am not asking for such experimental assessment, but for how these probes are expected to work under real-life conditions; especially with respect to the highlighted "on demand".

Response:
As shown in our previous response letter, we would like to emphasis that the time-dependent colour modulation can be controlled on demand before excitation.
We can design the time-dependent colour evolution process by tuning the chemical compositions or by using a core@shell structure for specific requirements of different applications. The UC and DS are generally generated under NIR and UV pumping sources, respectively. Thus, when combining the XEA with UC and DS, after triggering the XEA, the time-dependent colour evolution process can be controlled by changing the excitation wavelength or power (NIR and UV).
Considering the afterglow decay rate can be accelerated by elevating temperature, the time-dependent colour evolution processes will be influenced by temperature.
Similarly, the other environmental parameters, which can disturb the behavior of electrons releasing from traps, might influence the time-dependent colour evolution processes as well. As a result, it is of significance to further study these issues for practical applications.
The previously related respond contents are copy below.
To show that the time-dependent colour modulation can be controlled on demand, a schematic illustration is plotted revealing the colour variations. As shown in Figure   R2, the afterglow contains green and red colors (a), and the green one decays slower than the red one (b). In this case, if the initial intensity of the red colour is much stronger than the green one, then the output afterglow will be changed from red to green (c); if the initial intensity of the red colour is similar to the green one, then the output afterglow will be changed from yellow to green (d); if the initial intensity of the red colour is weaker than the green one, then the output afterglow will be green (e). This is only a simplified illustration. In our case, the Tb (green), Dy (cyan) and Pr (red) activators exhibit different afterglow colours and different decay rates. Thus, through a combination of different activators and tuning their relative initial XEA intensities (i.e., tuning the [Na]/[RR] ratio and the Gd 3+ doping content), the time-dependent colour change can be controlled on demand. Similarly, the DS and UC intensities can be tuned by the excitation power. Hence, for the core/shell/shell NPs that exhibit XEA, DS and UC emissions, the output colours can be modulated as well.

Figure R2
Schematic illustration of XEA colour variations at different conditions.
Comment: Potential application of the materials -bioimaging and biosensing as well as drug release monitoring: Taking into account that the emitted light falls into the visible spectral region, bioimaging of thicker samples / in-vivo seems not to be feasible to me. The suggested PDT, in contrast, seems to be promising to me, given that the samples could be excited outside the body and the PDT take affect afterwards, once admitted to the body.
Response: According to the reviewer's helpful suggestion, the examples of bioimaging and biosensing are removed from the main text. The original sentence has been corrected to "…which may find promising applications in biomedicine (i.e., photodynamic thereapy)".

Comment:
Experimental Section: was the precursor mixture stirred to remove water under ambient pressure or vacuum?
Response: Pure N2 was used throughout for the preparation of NPs. At 150 o C, water was removed together with the flowing N2.
The following content has been added in the Experimental Section. 10 The mixture was heated at 150 o C for 30 min to remove water from the solution under N2 atmosphere.
Comment: SI line 77: the amount of NaOA should be given in mol as for the other compounds.

Response:
The "NaOA (8.21 mmol)" has been added in the Experimental Section.
Comment: Size distribution histograms should include a y-axis to show how many NPs were considered in the size count.
Response: As suggested, the Counts have been added on the y-axis in the size distribution histograms in Figures S1, S24 and S42.  Comment: Figure S5a and b: These are data for the ratio of 10 or 12.5?
Response: Figure S5a and b are are data for the ratio of 12.5. The captions shown in Comment: Figures S9 and S10, b: what was the "normalized integral intensity" normalized to? One might expect a max value of 1 in case of a normalized data representation.

Response:
The integral intensity of the NaLuF4: Tb NPs (without Gd) was normalized to 1 in the Figures S9 and S10.
The sentence of "The case of 0 mol% is normalized to 1." has been added in the captions of Figures S9 and S10.
Comment: Schematic illustrations of crystal lattices: addition of a color code for the different ions might be helpful.

Response:
As suggested, the colour code of Na has been changed to another colour code in the Figure 3d.

REVIEWERS' COMMENTS
Reviewer #2 (Remarks to the Author): The careful revision by the authors has further clarified the manuscript. ICP data in Table S1 confirm nicely the relative amounts of Lu, Gd and Tb relative to each other, confirming a close match with the anticipated dopant % for Gd and Tb of 15%.
Here, I am wondering why the Na/RE ratio is calculated assuming a 100% RE concentration for each sample. One would expect slight changes in the ppm (or mol/L) of Gd, Tb and Lu from sample to sample. Why not dividing mol/L of Na by mol/L of RE (data as obtained by ICP)? Maybe such calculations were considered to obtain the shown [Na] relative to 100% RE. If so, okay.